In this research, the scope and limitation of two novel five- membered carbocyclic ring-forming reactions will be studied. In one of these reactions, and alpha, beta-unsaturated acyliron complex is combined with an allylstannone to form a five- membered ring. In the other reaction, a cyclopropylcarbene- chromium complex combines with an alkyne to produce beta- methoxycyclopentenone derivatives. We will show how this reaction can be used to synthesize pharmaceutically-interesting compounds such as triquinanes, prostaglandins, and cephalotaxine. The five-membered carbocyclic ring system is a very common structural feature in many pharmaceutically-interesting natural products. Development of cycloaddition approaches to five- membered rings will allow chemists to develop highly convergent synthesis to these highly useful compounds. As a result, the research presented in the proposal will be a great asset to the fields of organic chemistry and medicinal chemistry.